The present invention relates to a machine tool like a lathe and a grinding machine and so on.
In general, with regard to a machine tool like a lathe as shown in FIG. 7, in case that the feed control of a tool base 51 is carried out by a control unit 50 like NC units, the semi-closed loop control using the feedback signal F/B which is obtained from an encoder 53 of a servomotor 52 is adopted. The drive of the servomotor 52 is transmitted to the tool base 51 through a ball screw 56.
It is usually the case that a mechanical origin (Om) in the control unit 50 is set to position the location where the tool base 51 is the most far from a headstock 54. FIG. 8 shows the mechanical origin (Om) that is the origin in the coordinate of the feedback signal F/B by the encoder 53.
However, the thermal expansion is generated in the ball screw 56 as the cutting heat by the process and the other part heat by the machine operation is generated. Therefore, the semi-closed loop control has a substantial margin of error. In other words, a process origin (Ow) as shown in FIG. 8 is the center of a work (W) which is gripped by a spindle 55 (FIG. 7) and is positioned far from the mechanical origin (Om) that is the origin of the feedback signal F/B, so that the substantial margin of error comes out even if the feedback control is carried out, if there are the thermal expansion of the ball screw 56 and the mechanical deformation. The high accuracy amendment is difficult though the thermal expansion is amended, so that there is a limit to improve the process accuracy.
In order to improve the process accuracy, the closed loop control is also adopted by providing a linear sensor 57 as shown in the dash-dot line in FIG. 7. The linear sensor 57 is attached along the slide surface of the machine, for example a guide of the tool base (not shown in the drawings) in a bed 58. According to this, the position of the tool base 51 can be detected directly without acting on the thermal expansion of the ball screw 56.
However, as shown in the same drawings with emphasizing the displacement, in case of the disturbance like the minute lean in the headstock 54, the error becomes big even by the closed loop control in the same drawings. The above-mentioned disturbance is difficult to be amended by the control as it happens by the cutting heat, the load condition and the surroundings and the like, and the displacement is not fixed.
It is an object of the present invention to provide a machine tool that the process accuracy does not deteriorate even if the accuracy of the machine itself changes to some extent.
It is another object of the present invention that the high accuracy process mentioned above is carried out in a simple structure.